1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method of determining a position in a wireless communication system and apparatus thereof.
2. Discussion of the Related Art
First of all, a frame structure in a wireless communication system is described with reference to FIG. 1 as follows.
FIG. 1 is a diagram for a frame structure of LTE (long term evolution) system.
Referring to FIG. 1, a single frame is constructed with 10 subframes. Each of the subframes includes a pair of slots. A time taken to transmit one subframe is called a transmission time interval (hereinafter abbreviated TTI). For instance, a single subframe amounts to 1 ms and a single slot amounts to 0.5 ms.
One slot includes a plurality of OFDM (orthogonal frequency division multiplexing) symbols. In this case, the OFDM symbol can be called SC-FDMA symbol or symbol duration.
One slot includes 7 or 6 OFDM symbols in accordance with a length of a cyclic prefix (hereinafter abbreviated CP). In LTE system, cyclic prefixes can be classified into a normal CP and an extended CP. In case of using a normal CP, a single slot includes 7 OFDM symbols. In case of using an extended CP, a single slot includes 6 OFDM symbols. And, the extended CP is used in case that a delay spread is big.
FIG. 2 is a diagram of a slot structure in LTE.
Referring to FIG. 2, a signal transmitted in each slot can be depicted using a resource grid constructed with NDLRB×NDLSC subcarriers and NDLsymb OFDM (Orthogonal Frequency Division Multiplexing) symbols. In this case, the NDLRB indicates the number of resource blocks (RBs), the NDLSC indicates the number of subcarriers configuring a single RB, and the NDLsymb indicates the number of OFDM symbols in a single slot.
In the following description, a method of determining a position of a User Equipment according to a related art is explained.
First of all, the demand for a positioning method of a User Equipment is rising due to various applications in a real life. The positioning method of the user equipment can be mainly classified into a global positioning system (GPS) based scheme and a terrestrial positioning based scheme.
The GPS based scheme measures a position of a user equipment using satellites. Yet, the GPS based scheme requires reception signals from at least four satellites and is not applicable to an indoor environment.
The terrestrial positioning based scheme measures a position of a user equipment using a timing difference between signals from base stations and requires reception signals from at least three base stations. Although position estimating performance of the terrestrial positioning based scheme is inferior to that of the GPS based scheme, the terrestrial positioning based scheme is applicable to almost every environment. The terrestrial positioning based scheme estimates a position of a User Equipment using a synchronization signal or a reference signal. And, the terrestrial positioning based scheme can be defined as a following terminology per standard.
First of all, the terrestrial positioning based scheme is defined as OTDOA (Observed Time Difference of Arrival) in UTRAN (UMTS Terrestrial Radio Access Network). Secondly, the terrestrial positioning based scheme is defined as E-OTD (Enhanced Observed Time Difference) in GERAN (GSM/EDGE Radio Access Network). Thirdly, the terrestrial positioning based scheme is defined as AFLT (Advanced Forward Link Trilateration) in CDMA 2000.
FIG. 3 is a diagram for one example of downlink OTDOA as a sort of a terrestrial positioning based scheme used by 3GPP standards.
Referring to FIG. 3, since a User Equipment performs a reference clock with reference to a subframe transmitted from a current serving cell, signals received from neighbor cells differ from each other in TDOA (Time Difference Of Arrival).
FIG. 4 is a diagram for one example of a positioning method of a User Equipment (hereinafter abbreviated UE) using OTDOA.
Referring to FIG. 4, a method of determining a position of a UE is generally performed using a common reference signal (CRS) or a synchronization signal (e.g. primary synchronization signal/secondary synchronization signal: PSS/SSS). Alternatively, the method of determining a position of the UE can define and use a positioning reference signal (hereinafter abbreviated PRS) dedicated to LCS (location service). A user equipment finds a difference between a time taken to receive a signal from a single reference base station and a time taken to receive a signal from each of a plurality of neighboring base stations using reference or synchronization signals received from the single reference base station and a plurality of the neighboring base stations and then transmits the found time difference to an enhanced-serving mobile location center (E-SMLC). Subsequently, the E-SMLC is able to calculate a position of the UE by solving a linear equation using Taylor Series Expansion.
A location center (e.g. E-SMLC) is able to make a request for OTDOA information, which is required for calculating a position of a UE, to the base station. FIG. 5 is a diagram for exchanging OTDOA information between a location center and a base station.
Referring to FIG. 5, a location center transmits an OTDOA information request message to a base station. Having received the OTDOA information request message, the base station transmits an OTDOA information response message including OTDOA cell information to the location center. In this case, the OTDOA cell information contains a PRS configuration index of the base station, an SFN initialization time, a PRS muting configuration and the like.
Subsequently, the UE receives OTDOA information of a reference base station and OTDOA information of a plurality of neighboring base stations from the serving base station. In this case, the OTDOA information contains a PRS configuration index, a PRS muting configuration and the like.
In particular, the PRS configuration index indicates the information on a timing point of transmitting a PRS (positioning reference signal). Namely, the UE can acquire number of frame and number of slot, in which the base station transmits the PRS, from the PRS configuration index.
The UE is synchronized with a serving base station and is aware of a system frame number (hereinafter abbreviated SFN) of the serving base station only.
Yet, since the PRS configuration index is configured to match an SFN of the reference or neighboring base station that transmits the PRS, the UE should be aware of the SFN of the reference or neighboring base station.
In case of a synchronous network having transmission synchronization matched between base stations or a partially aligned synchronous network, since the UE is able to estimate an SFN boundary of the reference base station or neighboring base station with reference to the serving cell, it does not cause a serious problem. On the contrary, in case of an asynchronous network having transmission synchronization mismatched between base stations, the UE is able to receive the PRS from the reference base station or the neighboring base station only if aware of the SFN information of the reference or neighboring base station.
In case that the UE is not aware of the SFN information of a signal received from the reference or neighboring base station, the UE should obtain the SFN information by decoding P-BCH (primary-broadcast channel) of each of the corresponding base stations, which increases complexity of the UE. And, it causes a problem that P-BCH received from a base station having a low SINR of a reception signal has a low successful decoding rate.
Generally, since a reference cell is a cell to become a reference of TDOA, it is highly probable that a cell having good geometry will be set as a reference cell. Hence, a successful decoding rate of P-BCH can be high. Yet, the successful decoding rates of P-BCH of neighbor cells may be low. For instance, since Es/Iot reference of a reference cell for OTDOA and Es/Iot reference of a neighbor cell for OTDOA in the definition by TS 36.133 are −6 dB and −13 dB, respectively, it is difficult for a UE to successfully decode P-BCH of the neighbor cell.
In particular, according to a related art, since a UE is synchronized with a serving base station, it causes a problem that efficiency in receiving PRS from a reference cell or a neighbor cell is lowered.
According to a related art, in which PRS muting information is defined with reference to SFN of a serving cell, a position on a subframe of muting is calculated with reference to the SFN of the serving cell. Hence, a PRS configuration index is configured to match SFN of a reference or neighbor cell that transmits PRS and the PRS muting information is defined with reference to the SFN of the serving cell, which causes a problem that the PRS configuration index and the PRS muting information conflict with each other.
According to a related art, in which PRS muting information is defined with reference to SFN of a reference cell, a position on a subframe of muting is calculated with reference to the SFN of the reference cell. Hence, a PRS configuration index is configured to match SFN of a reference or neighbor cell that transmits PRS and the PRS muting information is defined with reference to the SFN of the reference cell, which causes a problem that the PRS configuration index and the PRS muting information conflict with each other.
As mentioned in the foregoing description, according to the related arts, efficiency in receiving PRS from a reference or neighbor cell is lowered.